In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least an insulative photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by bringing an electrographic developer into contact with the latent image. If desired, the latent image can be transferred to another surface before development.
In latent-image formation the imagewise discharge is brought about by the radiation-induced creation of pairs of negative-charge electrons and positive-charge holes, which are generated by a material (often referred to as a charge-generation material) in the electrophotographic element in response to exposure to the imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic potential and the type of materials included in the electrophotographic element, either the holes or the electrons that have been generated migrate toward the charged surface of the element in the exposed areas and thereby cause the imagewise discharge of the initial potential. What remains is a non-uniform potential constituting the electrostatic latent image.
Among the various known types of electrophotographic elements are those generally referred to as multiactive elements (also sometimes called multilayer or multi-active-layer elements). Multiactive elements are so named, because they contain at least two active layers, at least one of which is capable of generating electron/hole pairs in response to exposure to actinic radiation and is referred to as a charge-generation layer (hereinafter also referred to as a CGL), and at least one of which is capable of accepting and transporting charges generated by the charge-generation layer and is referred to as a charge-transport layer (hereinafter also referred to as a CTL). Such elements typically comprise at least an electrically conductive layer, a CGL, and a CTL. Either the CGL or the CTL is in electrical contact with both the electrically conductive layer and the remaining CGL or CTL. The CGL comprises at least a charge-generation material; the CTL comprises at least a charge-transport material (a material which readily accepts holes and/or electrons generated by the charge-generation material in the CGL and facilitates their migration through the CTL in order to cause imagewise electrical discharge of the element and thereby create the electrostatic latent image); and either or both layers may additionally comprise a film-forming polymeric binder.
Many multiactive electrophotographic elements currently in use are designed to be initially charged with a negative polarity and to be developed with a positively charged toner material. Usually, the arrangement of layers in such elements has the CGL situated between the CTL and the electrically conductive layer, so that the CTL is the uppermost of the three layers, and its outer surface bears the initial negative charge (although in some cases there may be a protective overcoat over the CTL which bears the initial charge). Such elements contain a charge-transport material in the CTL which facilitates the migration of positive holes (generated in the CGL) toward the negatively charged CTL surface in imagewise exposed areas in order to cause imagewise discharge. Such material is often referred to as a hole-transport material. In elements of that type a positively charged toner material is then used to develop the remaining imagewise unexposed portions of the negative-polarity potential (i.e., the latent image) into a toner image. Because of the wide use of negatively charging elements, considerable numbers and types of positively charging toners have been fashioned and are available for use in electrographic developers. Conversely, fewer high quality negatively charging toners are available.
However, for some applications of electrophotography it is more desirable to be able to develop the surface areas of the element that have been imagewise exposed to actinic radiation, rather than those that remain imagewise unexposed. For example, in electrophotographic printing of alphanumeric characters it is more desirable to be able to expose the relatively small percentage of surface area that will actually be developed to form visible alphanumeric toner images, rather than waste energy exposing the relatively large percentage of surface area that will constitute undeveloped background portions of the final image. In order to accomplish this while still employing widely available high quality positively charging toners, it is necessary to use an electrophotographic element that is designed to be positively charged. Thus, positive toner can then be used to develop the exposed surface areas (which will have relatively negative electrostatic potential after exposure and discharge, compared with the unexposed areas, where the initial positive potential will remain).
A multiactive electrophotographic element can be designed to be initially positively charged and still have the layer arrangement wherein the CGL is situated between the CTL and the electrically conductive layer. However, such an element must contain an adequate electron-transport agent (i.e., a material which adequately facilitates the migration of photo-generated electrons toward the positively charged insulative element surface) in its CTL. Unfortunately (and analogous to the situation with positive and negative toners), many materials having good hole-transport properties have been fashioned for use in electrophotographic elements, but relatively few materials are known to provide good electron-transport properties in electrophotographic elements.
Fortunately, a multiactive electrophotographic element can be designed to be positively charged while containing only a good hole-transport material in its CTL (rather than an electron-transport material), if a different arrangement of layers is employed, namely, wherein the CTL is situated between the CGL and the electrically conductive layer. Such elements are sometimes referred to as inverse multiactive elements and are known in the art (see, for example U.S. Pat. No. 4,175,960, FIGS. 6a through 6d). In inverse multiactive elements that are initially positively charged, the positive charge resides at the uppermost surface of the CGL, which is the uppermost of the three layers. Upon imagewise exposure of the inverse element, electron/hole pairs are created as usual in the imagewise exposed portions of the CGL, but in this case the photogenerated electrons migrate to the positively charged upper surface of the CGL, while the photogenerated holes migrate through the lower surface of the CGL and then downwardly through the CTL (their migration being facilitated by the hole-transport material in the CTL) to the electrically conductive layer.
It is also known that in multiactive elements that employ hole-transport materials in the CTL, it can also be beneficial to additionally include a hole-transport material in the CGL (in addition to the charge-generation material that must be there) in order to facilitate the migration of photo-generated holes through the CGL. See, for example, U.S. Pat. No. 4,175,960.
As mentioned above, many useful hole-transport materials are known in the art. See, again, for example, U.S. Pat. No. 4,175,960. Two types of such known useful hole-transport materials, among many others that are known, are triarylamines and polyarylalkanes.
The term, "triarylamine," as used herein is intended to mean any chemical compound containing at least one nitrogen atom that is bonded by at least three single bonds directly to aromatic rings or ring systems. The aromatic rings or ring systems can be unsubstituted or can be further bonded to any number and any types of substituents.
The term, "polyarylalkane," as used herein, is intended to mean any chemical compound containing an alkane group having at least one of its alkane carbon atoms bonded by at least two single bonds directly to aromatic rings or ring systems (which aromatic rings or ring systems can be unsubstituted or can be further bonded to any number and any types of substituents), with the proviso that such compound does not contain a nitrogen atom that is bonded by at least three single bonds directly to aromatic rings or ring systems (i.e., such compound does not contain a triarylamine moiety).
The present inventors have recognized a number of drawbacks associated with inverse multiactive elements that are intended to be positively charged on the outer surface of the CGL and that have a hole-transport material, in addition to a charge-generation material, in the CGL.
For example, the present inventors have recognized that some charge-transport materials are very efficient at transporting positive-charge holes generated by charge-generation materials but are also significantly susceptible to injection of positive charge from the positively charged outer surface of the CGL (sometimes also referred to as "positive-surface-charge injection"), and they have recognized that most triarylamines are charge-transport materials of this type. The present inventors have further recognized that if an inverse multiactive element, as previously described, contains this type of charge-transport material (in addition to a charge-generation material) in its CGL, the element will exhibit less than desirable charge uniformity and lower and less stable than desirable charge acceptance upon cycling. The term, "charge uniformity" refers to the degree of variation of the level of potential at various points on the initially charged electrophotographic element, before imagewise exposure and discharge; a low degree of potential variation among various points corresponds to high charge uniformity and vice versa. The phrase, "charge acceptance upon cycling," refers to the capability of the element to be initially charged to the desired level of potential at the beginning of each cycle of its normal operation (a cycle being the sequence of operation comprising initially uniformly charging the element, then exposing the element imagewise to actinic radiation to form the electrostatic latent image, followed by erasure of the remaining potential on the element to prepare it for the next cycle of operation) after a plurality of such cycles of operation have been carried out. "Low charge acceptance upon cycling," means that, at least after a number of cycles of operation, the element has a relatively poor capability of being initially charged to the desired level of potential. "Less stable charge acceptance upon cycling," means that the capability of the element to be initially charged to the desired level of potential changes very significantly after a plurality of cycles of operation. The present inventors believe that relatively low and less stable positive-charge acceptance upon cycling are reliable indicators that the charge-transport material in the CGL has a relatively high degree of susceptibility to positive-surface-charge injection.
Also, for example, the present inventors have recognized that some other charge-transport materials are less efficient at transporting positive-charge holes and less susceptible to positive-surface-charge injection than, e.g., triarylamines, and they have recognized that most polyarylalkanes (as defined above) are charge-transport materials of this type. The present inventors have further recognized that if an inverse multiactive element, as previously described, contains this type of charge-transport material (in addition to a charge-generation material) in its CGL, the element will exhibit lower than desirable photosensitivity and, after one or more cycles of operation, higher than desirable residual potential. The term, "photosensitivity" (sometimes also commonly referred to as electrophotographic speed) refers to the amount of incident actinic radiant energy to which the element must be exposed in order to achieve the desired degree of discharge of the initial potential to which the element was initially charged. The lesser the amount of radiant energy required for such discharge is, the higher is the photosensitivity, and vice versa. The term, "residual potential," refers to the final potential to which the element can be driven by the erasure step (e.g., by exposure to excess amounts of actinic radiation) at the end of a cycle of operation. Lower residual potential is more desirable, because if the residual potential is higher than the level of potential intended to be reached by discharge of the element in areas of maximum imagewise exposure during latent image formation, that intended level of potential will not be reached, and the latent image will constitute an inaccurate record of the image intended to be represented.
The present inventors were faced with the problem of providing an inverse multiactive electrophotographic element that avoids or minimizes the drawbacks discussed above, i.e., an element that will exhibit, during its normal operation, relatively high charge uniformity, relatively high and stable charge acceptance upon cycling, relatively high photosensitivity, and relatively low residual potential.